The present invention relates in general to a method and apparatus for treating a material such as substrate, wafer or the like by utilizing a plasma. More particularly, the present invention is concerned with a method and apparatus for etching a substrate by sputtering at a high speed by utilizing a plasma without damaging or injuring the substrate. The invention is further concerned with a method and apparatus for depositing a film by a plasma CVD process at a high speed.
There is known in the prior art such a sputter etching apparatus in which an etching electrode having a substrate disposed thereon is mounted within a vacuum chamber. By applying a high-frequency power to the electrode, a plasma is produced over the substrate, wherein ions in the plasma are caused to impact against the substrate under acceleration by the electric field generated under the potential appearing on the substrate, whereby the latter is etched by the sputtering. With this structure of the sputter etching apparatus, however, the plasma produced over or above the substrate is of a low density, as a result of which the sputter etching can proceed only at a low speed. When the high-frequency power applied to the etching electrode is increased in an effort to speed up the sputter etching, the ions will then impact against the substrate with increased energy because of a high voltage appearing on the electrode and substrate, giving rise to the possibility of the substrate being undesirably damaged or injured. As a means of increasing the sputter etching rate or speed with energy of the ions bombarding the substrate being maintained at a low level, there can be mentioned a method which is disclosed in Japanese Patent Application Laid-Open No. 74436/1985 (JP-A-60-74436). According to this known proposal, a plasma produced by applying a high-frequency power to the etching electrode is confined within a delimited space over the substrate by using a floating electrode in combination with a magnet for the purpose of preventing the plasma from diffusing radially outwardly while increasing the number of ions in the plasma and hence the density thereof. With this arrangement, the voltage making appearance on the etching electrode can certainly be prevented from being increased relative to the power applied to the electrode. It is however noted that the increase in the density of plasma is not proportional to the increase in the power applied to the etching electrode at a definite one-to-one ratio. Consequently, when the applied high-frequency power is increased in order to further speed up the etching, the voltage appearing on the electrode is increased to such an extent that damage or injury can occur, to the substrate.
Another plasma treatment apparatus of the prior art is disclosed in JP-A-57-26441. In the case of this known apparatus, there is provided, in combination with the reaction chamber, a preliminary excitation chamber into which only a gas difficult to activate is supplied for preliminary excitation, the gas thus activated is then introduced into the reaction chamber together with a gas easy to activate. In this way, concentration of the activated gases which partake straightforwardly in the formation of a film can be increased without need for applying a high power to the electrode within the reaction chamber, whereby the material to be treated such as a substrate can be protected against damage or injury. Besides, by regulating the electric power supplied to the preliminary excitation chamber and the reaction chamber, the ratio of composition of the film formed by deposition can be controlled for ensuring a high quality of the film. However, this known apparatus suffers from a shortcoming that the activation of the gas difficult to activate is inadequate because excitation of the gas of this species relies on a high-frequency discharge.
A further plasma treatment method in the prior art is disclosed in JP-A-57-167631. According to this known technique, the preliminary excitation of a reactant gas is realized by utilizing microwave energy. By using a microwave (usually having a frequency of 2.45 GHz) in place of a high-frequency signal (normally of 13.56 MHz) as proposed, the plasma density is increased by one to two orders of magnitude (approximately up to 10.sup.11 /cm.sup.3) because the ratio at which the reactant gas is excited due to impact of electrons in the plasma is increased, whereby concentration of the activated gas is enhanced.
However, in the case of the known apparatus just mentioned above, a long distance intervenes between the activating chamber and the reaction chamber. Besides, a passage for introducing the activated gas into the reaction chamber is provided at an offset position. Consequently, difficulty is encountered in introducing the gas activated through the preliminary excitation onto a substrate under treatment uniformly without lowering concentration of the activated gas. Such being the circumstances, it is difficult to carry out the treatment uniformly at a high speed, although it is admitted that a film of improved quality can be formed on the substrate.
As will be seen from the above discussion, according to the prior art techniques, in these endeavors consideration is not paid to the uniform formation of a high quality film on a substrate at a high speed without injuring the substrate. In other words, according to the known techniques such as mentioned above, the quality and/or uniformity of the film as formed will degrade when the treating rate or speed is increased. Thus an attempt for forming a uniform film of high quality is inevitably accompanied with lowering in the treating speed in the case of the hitherto known techniques.